In recent years, as a drive power source for electronic equipment, a power storage device having high voltage and high energy density has been desired. Lithium ion batteries, lithium ion secondary batteries and lithium ion capacitors have been expected as power storage devices for this use.
Electrodes used in such power storage devices are generally manufactured by coating a mixture of an electrode active material and polymer particles which serve as a binder on the surface of a collector and drying it. Characteristic properties required for the polymer particles include bonding ability between electrode active materials, adhesion ability between an electrode active material and a collector, abrasion resistance in the step of taking up an electrode, and powder fall-off resistance that fine powders of the electrode active material are not produced from an electrode composition coating layer (to be referred to as “electrode active material layer” or simply as “active material layer” hereinafter) by the subsequent cutting step. When the polymer particles satisfy these requirements, the degree of freedom for the method of folding the obtained electrode and for designing the structure of a power storage device such as the setting of the winding radius becomes high, thereby making possible the downsizing of the device. It is empirically found that, as for bonding ability between the electrode active materials, adhesion ability between the electrode active material layer and the collector, and powder fall-off resistance, their performances are almost in proportion to one another. Therefore, in this text, they may be comprehensively expressed by using the term “adhesion” hereinafter.
The required performance of the binder material is becoming higher and higher. When the material is used in a positive electrode, it must have high resistance to oxidation and when it is used in a negative electrode, it must have high resistance to reduction. In addition, the binder material must have a fine balance between the property of facilitating the movement of an electrolyte by swelling appropriately through contact with an electrolytic solution and the property of preventing the above swelling from causing the increase of electrode resistance and the reduction of adhesion. Further, in recent years, a power storage device capable of high-speed discharge which can ensure rapid acceleration when it is mounted as a drive power source for electric cars has been desired.
Under the above situation, in the prior art, various proposals were made to adjust the affinity for an electrolytic solution of a binder material so as to improve adhesion, charge/discharge characteristics and durability. For example, technology for introducing a cyclohexyl group into a polymer which is a binder material (JP-A 2012-104406 (Patent Document 1)) and technology for introducing a nitrile group (JP-A08-287915 (Patent Document 2) and JP-A2006-48932 (Patent Document 3)) are proposed.
Even with the above prior art, high-speed charge/discharge characteristics which enable a drive power source to be mounted on an electric car are not attained. That is, the technology of Patent Document 1 is based on an idea of enhancing the time durability of a power storage device by improving the electrolytic solution resistance of the binder material. However, according to this technology, since the affinity for an electrolytic solution of the binder component is impaired, the required level of high-speed discharge characteristics is not attained. Meanwhile, the technology of Patent Documents 2 and 3 is aimed to attain high-speed discharge characteristics by improving the affinity for an electrolytic solution of the binder material. However, since this material has excessively high swellability when it comes into contact with an electrolytic solution, its deterioration during use or storage of the power storage device at a high temperature is large. Therefore, it has a problem with durability.
Thus, in the prior art, it is considered that there is a trade-off relationship between the durability of a power storage device and charge/discharge characteristics (especially high-speed discharge characteristics).
To realize the miniaturization of a power storage device, a separator film for separating a positive electrode from a negative electrode must be reduced in thickness. Since the distance between the positive electrode and the negative electrode becomes short by making the separator thin, the height of the power storage device can be reduced. However, when the distance between the positive electrode and the negative electrode becomes short, the potential of the occurrence of a short circuit becomes high. Especially in a power storage device making use of a metal ion conductor (for example, a lithium ion), dendrite is produced on the surface of an electrode by repetitions of charging and discharging with the result that a short circuit occurs more easily. Dendrite is a metal precipitate formed from a metal ion and generally grows into a needle form. Therefore, dendrite may grow through the separator which is a porous film and may cause a short circuit in a power storage device when it reaches the surface of an opposite electrode.
For the above reason, when the distance between the positive electrode and the negative electrode becomes short by the reduction of the thickness of the separator film, the risk of a short circuit caused by dendrite becomes high. To eliminate this phenomenon, an attempt is being made to form a porous film on the surface of the separator so as to enhance the penetration resistance of the separator. As the material forming the above porous film, a material comprising inorganic fine particles and a resin binder selected from polyimide resin, polyamide resin and polyamide-imide resin is proposed by WO2009/041395 (Patent Document 4) and JP-A 2009-87562 (Patent Document 5), and a material comprising metal oxide fine particles and a binder containing a fluorine-based resin and rubber-based resin is proposed by JP-A 2009-54455 (Patent Document 6).
In the technologies of Patent Documents 4 to 6, a certain effect of preventing a short circuit caused by dendrite in a power storage device making use of a metal ion conductor is recognized. However, since these protective films are unsatisfactory in terms of the permeability and retainability of an electrolytic solution, the internal resistance of a power storage device rises, thereby impairing charge/discharge characteristics.